'I've been utterly and completely castigated from time to time," says Diana Rigg, transfixing me with her famously intense, unflinching brown gaze. "Once, when I was playing a nude scene in an indifferent play in New York, a critic wrote: 'Diana Rigg is built like a brick basilica with too few flying buttresses.' Do you think that's fair?"
I should say no, of course. I've glimpsed enough of her embonpoint in the seduction scene of Who's Afraid of Virginia Woolf? to appreciate that Rigg's 58-year-old buttresses are still magnificently airborne. But I don't want to sound too sycophantic, and, besides, I'm rather keen to discover whether there's a sense of humour beneath that dauntingly poised veneer. "I'm not sure," I mumble, "I haven't seen you naked."
There's a split-second pause, which leads me to fear I've gravely misjudged the situation. Up until now, Rigg hasn't exactly been easy going: lofty, clipped pronouncements on the "paramount" importance of diction and verse-speaking; close textual analysis of Edward Albee's masterpiece; but precious little mention of The Avengers or Theatre of Blood. Then suddenly, I'm rewarded with a long, delicious, Marlboro-stained laugh. Thank heavens! Emma Peel hasn't gone and turned into an earnest luvvie.
Just to make sure, I ask whether she shares director Trevor Nunn's abhorrence of the "L" word. "I don't get quite as exercised as he does because I think it applies to him more than it does to me," says Rigg. "It's mega tease and one should stay at the bottom of the pool."
The fly-fishing metaphor is the clincher. Actresses of Dame Diana Rigg's stature are meant to be virulently political and right-on. They are not meant - as she does in her spare time - to enjoy patrician sports that involve wanton cruelty to trout. Nor, as she also does, should they say kind things about that traditional bÍte noire, the theatre critic.
"I don't mean to be oily," says Rigg, who once published an anthology of theatre reviews, "but critics are very much part of the theatre. There are those who have a knowledge and passion for the theatre, and those who don't. But they contribute to the body of literature that's written about the theatre. We now depend on critics of the past to give us a flavour of what was going on then."
The beneficent feeling is mutual. It has been a long time since any reviewer had an unkind word to say about Rigg. In 1993, her Medea was universally acclaimed in London and on Broadway; so too was her Mother Courage last year at the National; and this year, she completed her hat-trick with a virtuoso account of Martha, the drunken hysteric in Who's Afraid of Virginia Woolf?
"Her Martha is a truly fearsome creature," declared Irving Wardle in The Sunday Telegraph; "Loud, vulgar, sexy and often devastatingly funny," agreed The Daily Telegraph ; "Lustrous," said The Guardian; "A rare and shattering event," said the Daily Mail. Albee himself declared it one of the finest of the 100 or so productions he had seen.
Rigg may have given up reading her reviews, but she has had plenty of encouraging feedback from audiences at the Almeida Theatre, from which the sell-out production will soon be transferring to the West End. Much of its success, she believes, lies in the fact that it's genuinely funny.
Unlike the Richard Burton and Elizabeth Taylor movie. "I was disappointed that there was so little humour in the film," she says. "But it's one of the threads that knits George and Martha together. They love each other's sense of humour."
We're sitting in the Almeida's bar shortly before curtain-up. Through the window you can see a long queue of students waiting for standby tickets. They're pretending not to gawp at the show's star. Rigg remains ravishingly beautiful: fine cheekbones, captivating brown eyes, an elegant auburn bob, set off by a black jumper and gold necklace.
Though she's now well used to those sneaky glances, she still squirms at the memory of the attention she received when, after three years with the Royal Shakespeare Company, she became The Avengers' karate-chopping sex kitten Emma Peel.
"It was frightening. I was used to strolling in the street without a second thought. And suddenly everyone was looking at you wherever you went, nudging, winking."
Her smart theatrical friends insisted that quitting the RSC for such a lowly medium was a huge mistake. "'Nowadays everybody does it. In those days they were really snotty."
But she has few regrets. "It wasn't harmful. How could it be? It catapulted me to the kind of fame that would have taken 20 years' solid touring to achieve."
The main drawbacks were that Rigg acquired a reputation for being as cool as Emma Peel - "I'm the least cool person in the world," she insists - and that it stymied her film career. "I kept being offered gun-toting parts and I didn't really want that." Most of the plum movie roles, she says, went instead to Vanessa Redgrave or Glenda Jackson: "I came a very poor third."
Cinema, certainly, is one of the few acting areas in which Rigg has not excelled. Her best film, she thinks, was Theatre of Blood - a cod-Shakespearian horror flick in which she played Vincent Price's daughter. This doesn't say much for the rest, which include On Her Majesty's Secret Service (with the forgettable George Lazenby as James Bond) and The Assassination Bureau (with Oliver Reed).
But theatre was always Rigg's true love. She was never happier than during her early days in rep, when she acted as an assistant stage manager and played walk-on parts in order to get an Equity card.
"It was the nicest feeling in the world. It was my first job and I had nowhere to go but up. Of course, I could have stayed at the bottom. But that's inconceivable when you're 19 and full of natural optimism."
After a year's apprenticeship at the RSC - "I carried spears, literally, as an amazon in A Midsummer Night's Dream" - she was given a three-year contract with the RSC, where she played Bianca, Helena, Lady Macduff and Cordelia to Paul Scofield's Lear.
"A brilliantly skilled and delicious actress," observed Laurence Olivier. "Anyone who doesn't find her devastatingly attractive must be an Outer Mongolian monk," said Douglas Fairbanks Jnr.
But life since then has not been without its pitfalls. There was Diana, her disastrous television show in Hollywood. There was her failed marriage to Israeli artist Menachem Gueffen, and another to Scottish landowner Archie Stirling (father of her 19-year-old daughter Rachel), which also ended in divorce after he had an affair with Vanessa Redgrave's younger daughter Joely Richardson (though the couple are now close again). There was also a bleak period in the Eighties when she didn't work for two years. The latter she puts down to looking after her daughter and turning down work. "I didn't pay my dues."
Today her diary is looking rather fuller. Who's Afraid of Virginia Woolf? transfers on Wednesday to the Aldwych Theatre, and this autumn she can be seen on television as Mrs Danvers in Rebecca (the first film to be based on the du Maurier novel, rather than the play), and as the heroine's mother in Moll Flanders.
"I want to spend more time playing character parts," she says. "I want to be the Denholm Elliott of the film world."
With those looks, unfortunately, this may be one of the few thespian ambitions beyond Rigg's considerable grasp.